Wisconsin WWTP becomes more energy-efficient by adding aeration technology
Driven by compliance with more stringent water treatment standards, the Fond du Lac (Wis.) Wastewater Treatment Plant leveraged an upgrade opportunity to improve beyond limiting ammonia effluent levels. The plant improved energy efficiency, reduced maintenance and operating costs, and eliminated bypasses into Lake Winnebago during high rainfall.
The upgrade was based on an average daily design flow of 9.84 mgd and a projected biological oxygen demand load of 22,500 lb/day. To contain costs, the utility used its existing 22-acre site and reused two 95-ft-diameter retired clarifiers, which also served as storage basins during peak flows to avoid untreated water from reaching Lake Winnebago. The more challenging aspect of the project was meeting the energy reduction targets despite having to double the hydraulic capacity and pump 50% more head.
After thorough analyses, Xylem was selected to deliver a comprehensive aeration solution utilizing Flygt pumps and Sanitaire fine-bubble aeration equipment. Twenty-six Flygt recycle pumps and more than 1,900 9-in.-diameter Sanitaire diffusers (five grids per tank) drove the energy-efficient activated sludge process.
The plant’s Flygt pumps and mixers operate in two capacities. Two 85-hp pumps cover the low end of the influent flow range (up to 5 mgd) while four 215-hp pumps, in combination with the two smaller pumps, provide the 50-mgd peak capacity for the pumping station. Variable-speed drives minimized energy consumption by synchronizing the combination of pumps and their operating speed with the flow conditions. The energy-saving features of the self-cleaning N-pumps yielded the plant additional cost savings.
The Flygt equipment performs a variety of essential missions in the plant process. Mixers blend primary sludge with waste-activated sludge to facilitate pumping and digestion in the anaerobic digesters. Still others pump drainage from the primary sludge building sump as part of the plant’s wastewater collection system, and the two 1.7-hp pumps maintain a dry basement in the solids-processing building. Six 44-hp propeller pumps discharge the plant’s treated effluent to Lake Winnebago while three 27-hp propeller-type units pump storm water collected in a segregated retention basin to the Fond du Lac River.
Low-energy, ultra-low head Flygt pumps enable the plant to comply with permit limits for ammonia with greater energy efficiency. By recovering oxygen from bound nitrates, the plant reduces aeration consumption and utilizes the energy-efficient Flygt pumps to transfer the nitrate recycled from the aeration zone into the anoxic zone. For the remaining aeration requirement, energy consumption is minimized using state-of-the-art, high-efficiency blowers and Sanitaire fine-bubble diffused aeration.
Alliant Energy, the electric utility serving the area, honored the Fond du Lac utility for adding advanced technology that saved 1.95 million kW-hours of electricity and 188,140 therms of natural gas—enough to power 212 average-size homes for one year. The plant also reduced its operating costs, including a reduction in personnel from 15 to six on one shift and a 20% reduction in overtime compared to previous levels.
While compliance is not an option, the choice of technologies is. Flygt pumps and mixers, along with Sanitaire fine-bubble diffused aeration, provide an energy-efficient option to achieve compliance and operational efficiencies.